v0.4.0

Changed

• The protocol library and the connection engine moved to a separate repository and crate,
  open-stomp (open_stomp), which the app now uses.
  It replaces crates/stomp-proto and the app's own device module, carries the full protocol
  documentation plus a library guide, and the capture-driven tests. Building the app now needs a
  checkout of open-stomp-lib next to this repository (CI, the release workflow and the Nix
  flake fetch it; CI and releases pin open-stomp-lib v0.1.0).
• Documentation split: the protocol and reverse-engineering docs (01–09) now live in
  open-stomp-lib and this
  repository links there. Its own docs are about the app: a user guide,
  building and architecture, and the README is
  rewritten around the app.